This proposal is for the Clinical Trials Statistical and Data Management Center (CTSDMC) in the Department of Biostatistics, College of Public Health at the University of Iowa for the Carotid Occlusion Surgery Study (COSS) and covers the activities of the CTSDMC. Dr. William Powers is submitting a separate proposal for the Clinical Coordinating Center (CCC) for this trial. The goal of this study is to test the hypothesis that extra cranial-intra cranial (EC/IC) arterial bypass by surgical anastomosis of the superficial temporal artery to the middle cerebral artery (STA-MCA) when added to best medical therapy can reduce by 40%, despite perioperative stroke and death, subsequent ipsilateral ischemic stroke (fatal and non-fatal) at two years in patients with symptomatic carotid artery occlusion and increased oxygen extraction fraction measured by positron emission tomography (PET). This is a randomized, non-blinded, controlled trial in 372 participants randomized equally to surgical and non-surgical treatment. Participants who fulfill clinical entry criteria and who have carotid occlusion in at least one carotid artery by vascular imaging will be eligible for enrollment to undergo PET. If PET criteria is met then arteriographic criteria must be met for the participant to be randomized. All randomized participants will receive the best available medical treatment with anti-thrombotic agents and risk factor intervention to prevent subsequent stroke. Participants are followed for 24 months at 3 month intervals. The CTSDMC brings over 30 years experience in the design and implementation of clinical and epidemiology studies and over 14 years experience serving as the data coordinating center for multi-center clinical trials. These studies include studies in neurology, nephrology, and diabetes. The CTSDMC staff has successfully developed and operated web based data entry systems and databases for multi-center trials and have a excellent record in successfully coordinating clinical trials.